The present invention relates to a biaxially oriented polyester film for magnetic recording media, particularly digital data storage tapes, and to a magnetic recording medium comprising the same as a base film.
Polyester films are used in a wide range of fields such as magnetic recording media and electric insulation because they have excellent thermal and mechanical properties. Along with marked progress made in the tape capacity and density of magnetic recording media, especially data storage magnetic recording media, higher and higher properties are now required of base films used in these media. In data storage magnetic recording media employing a linear track system such as QIC, DLT and large-capacity super DLT and LTO, the track pitch is made very narrow to increase the capacity of a tape. Therefore, when there is a dimensional change in the width direction of a tape, track dislocation occurs, thereby causing an error. This dimensional change is caused by temperature and humidity variations and also by time shrinkage in the width direction which occurs when the tape is caused to run repeatedly at a high temperature and high humidity under high tension. When this dimensional change is large, track dislocation occurs, thereby causing an error at the time of electromagnetic conversion. Particularly in the latter case, the dimensional change is marked when the thickness of a tape is reduced to increase the recording capacity of the tape. Therefore, this dimensional change must be improved. The time shrinkage in the width direction can be improved by increasing the Young""s modulus in a longitudinal direction of the base film. However, the upper limit of Young""s modulus in a transverse direction becomes smaller as the Young""s modulus in a longitudinal direction increases according to the relationship between the characteristic properties and film formation properties of a polymer. As a result, the dimensional change caused by temperature and humidity variations in the former case becomes large, thereby making it difficult to reduce the dimensional changes in both cases at the same time.
JP-A 5-212787 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a biaxially oriented polyethylene-2,6-naphthalene dicarboxylate film having a Young""s modulus in a longitudinal direction (EM) of 550 kg/mm2 or more and a Young""s modulus in a transverse direction (ET) of 700 kg/mm or more, a ratio of Young""s moduli (ET/EM) of 1.1 to 2.0, a shrinkage factor in a longitudinal direction of 0.02% or less when it is maintained at a relative humidity of 65% at 70xc2x0 C. under no load for 1 hour, a temperature expansion coefficient (xcex1t) in a longitudinal direction of 10xc3x9710xe2x88x926/xc2x0 C. or less and a humidity expansion coefficient (xcex1h) in a longitudinal direction of 15xc3x9710xe2x88x926/% RH or less.
International Publication WO99/29488 discloses a biaxially oriented polyester film having a thickness of less than 7 xcexcm, which has the following relationship among its thermal expansion coefficient xcex1t (xc3x9710xe2x88x926/xc2x0 C.) in a transverse direction, humidity expansion coefficient xcex1h (xc3x9710xe2x88x926/% RH) in a transverse direction and shrinkage factor P (ppm/g) in a transverse direction against a load applied in a longitudinal direction.
1 less than Pxe2x88x92(xcex1t+xcex1h)/10xe2x89xa610 
It is an object of the present invention to provide a biaxially oriented polyester film which has solved the above problems, hardly produces an error caused by track dislocation due to a dimensional change in the width of a tape and can improve output characteristics as a base film for digital data storage magnetic recording media of a linear track system.
It is another object of the present invention to provide a magnetic recording medium, especially a digital data storage magnetic recording medium comprising the biaxially oriented polyester film of the present invention as a base film.
Other objects and advantages of the present invention will become apparent from the following description.
Firstly, according to the present invention, the above objects and advantages of the present invention are attained by a biaxially oriented polyester film for magnetic recording media which has (1) a dimensional change between before treated and after treated in a direction perpendicular to a load application direction on the film plane of 0.40% or less when the film is treated at 49xc2x0 C. and 90% RH under a load of 2.7 kg per 1 mm2 of unit sectional area in a thickness direction of the film for 72 hours, (2) a crystallinity of 27 to 45%, (3) a temperature expansion coefficient xcex1t in a direction perpendicular to the above load application direction on the film plane of xe2x88x925xc3x9710xe2x88x926 to +20xc3x9710xe2x88x926/xc2x0 C. and a humidity expansion coefficient xcex1h in a direction perpendicular to the above load application direction on the film plane of +5xc3x9710xe2x88x926 to +20xc3x9710xe2x88x926/% RH, the value of (xcex1t +2 xcex1h) being +45xc3x9710xe2x88x926 or less, (4) a heat shrinkage factor in a direction perpendicular to the above load application direction on the film plane of 0 to 0.7%, and (5) a thickness of 3 to 7 xcexcm.
Secondly, the above objects and advantages of the present invention are attained by a magnetic recording medium comprising the biaxially oriented polyester film of the ""present invention and a magnetic layer formed on one side of the polyester film.